Madison joins McGrath's children James, 15, and Holly, 13, from his marriage to Jane, who died from breast cancer in 2008. In 2002, Glenn and Jane McGrath founded The McGrath Foundation, now one of Australia's leading breast cancer support organisations.

In June, McGrath told the Weekly he and Sara "couldn't be more delighted" to be adding to their family.

Having had a vasectomy in 2003, McGrath required a procedure to extract sperm. After two rounds of IVF, Sara became pregnant.

"Before I met Sara, that was it for me, I was never having kids again," he said.

"Even when I met Sara, I was adamant I wasn't going to have more kids. But then I was adamant I wasn't going to get married again either. Because I'm 45 now — it's not like I'm a spring chicken any more. So when the bub is 15, I'm going to be 60. And you do think about these things.

"But once I sat down and thought about it, and thought not only about the love we have for each other, but also the love we have for James and Holly, we thought we'd like to replicate that in our marriage."

Sara also had her doubts.

"To be honest with you, if I hadn't had the experience of being a step-mother to James and Holly, I'm not sure I would have had kids of my own," she said.

"But they are both just such lovely kids.

"There is no new family being created here. This baby is just an extension of the family we already have."

On Friday, the magazine tweeted: "Huge congratulations to Glenn and Sara McGrath who this afternoon welcomed their first child."